Forearm training should be taken just as seriously as any other body part if you want to develop a truly quality physique, according to Schwarzenegger. They are involved in nearly every exercise for the upper body, either by helping you grip a piece of equipment or by being a part of all pushing and pulling actions. So they get a lot of incidental training even when you are not specifically doing forearm exercises. In fact, anytime you flex the elbows or wrists, you put stress on the forearm muscles.
Good forearm development is necessary to create a championship physique, but forearm strength is just as important. Strong forearms allow you to train with heavier weights and, in exercises such as Chins and Cable Rows, in which the hand and wrist are generally the "weak link," give you the capacity to train hard and put more stress on other muscles.
As with other muscles, genetic structure is a factor in determining the potential size and strength of the forearms. The reason some forearm muscles seem to extend all the way to the hand, with almost no tendon intervening, is that person has an extremely long "muscle belly," the actual contractile part of the muscle-tendon structure. Muscle size is affected by the length of the muscle belly because mass is a product of volume, that is, three dimension rather than just one. So having two inches more length in forearm actually translates into a lot of extra potential when you consider what the increase in cubic measurement can be. Many bodybuilders constructed like this claim they do not need to do forearm training but get adequate results with exercises like heavy Barbell Curls.
However, when Schwarzenegger trained with Casey Viator, who had incredible forearm development, he was doing Barbell Wrist Curls with 155 pounds and Reverse Curls with 135 pounds. Sergio Oliva did endless sets of Reverse Curls on a preacher bench to get that enormous upper forearm development. Dave Draper did a lot of forearm training. So, even if you are genetically gifted with good forearms, this doesn't mean you don't have to train them.
It is also possible to have high forearms, that is, to have a relatively short muscle belly and a long tendon, limiting the cubic volume of the muscle mass. Most bodybuilders, Schwarzenegger included, are somewhere in between, with neither the full forearm structure of a Sergio Oliva nor impossibly high forearms. With this kind of forearm, it is possible to build the muscles up to where they are proportionate to the upper arm, but you have to train them hard to do so.